Current studies of monkeys will be continued and extended. I) The hand preference of cynomolgus monkeys during simple reaching will be determined in an attempt to confirm previous results in rhesus monkeys (27). Those results include: a tendency for greater handedness in individuals of older age, an initial trend toward increased handedness with repeated testing, and a later tendency for average handedness to reach an asymptote less than 100%. II) The hand preference of rhesus monkeys during simple reaching will be correlated with learning and relearning rates for a visual discrimination task. The data will be analyzed to determine whether there are differences in learning and relearning rates of: a) right and left handers, b) strong and weakly handed individuals, and c) individuals that succesively increase their handedness during further testing and those with no consistent increase. III) Animals with primary visual input restricted to one brain half often will prefer the contralateral hand when reaching for food even if this is not the hand preferred when primary input is not restricted to one side (23). Monkeys previously tested for hand preference will be retested after optic chiasm section. Monocular retesting will restrict primary visual input to one side of the brain. If the input were directed to the brain half ipsilateral to the preoperatively preferred hand, there should be a conflict between the animal's previous hand preference and that promoted by the restriction of visual input. It will be interesting to observe whether this results in an increased tendency to reach concurrently with both hands. IV) Split-brain animals with optic chiasm and forebrain commissures sectioned will be tested for monocular and binocular recall of a visual discrimination as reported previously for animals with chiasm section alone (30). Animals will be binocularly trained preoperatively and tested for recall postoperatively under monocular conditions. Comparison of postoperative recall through the right and left eyes.